1. Field of the Invention
The present invention relates to devices for recording and utilizing digital data, and in particular relates to a technique for preventing unauthorized use of digital data.
2. Description of the Related Art
The digitization of information is increasing in recent years. Since digitized information (hereinafter, “digital content”) is not only relatively easy to handle but also will not deteriorate with time, information such as audio and image data is becoming more and more digitized.
However, if digital content is copied, the copy will end up being precisely identical to the original. This provokes unauthorized acts such as illegal duplication or tampering of copyrighted information.
To discourage such unauthorized acts, a method is devised whereby storage mediums such as memory cards are respectively given unique numbers (hereinafter, “card IDs”) so that each storage medium stores only digital content encrypted using its own card ID.
According to this method, even when digital content which has been encrypted with a card ID of some memory card and stored on the memory card is copied onto another memory card, the copy of the encrypted digital content cannot be decrypted with a card ID of the latter memory card. Hence the digital content can be protected against such unauthorized copying.
Here, the digital content may be accompanied with restrictive information that restricts the use of the digital content to a limited number of times or to limited usage conditions.
By encrypting this restrictive information using the card ID and storing it onto the memory card in the same way as the digital content, the restrictive information can be protected from unauthorized copying as with the digital content.
Nevertheless, when the restrictive information contains information whose value changes over time, such as a limitation on the number of times the digital content can be used, then even if the restrictive information is encrypted with the unique card ID, it cannot be fully protected against falsification. That is to say, the user who receives the restrictive information of high value can make a backup copy of it and in later use replace the restrictive information of decreasing value with the backup copy.